Spacing of soil samples.

It is frequently specified that soil samples should be taken at intervals of 1.5 m and at each change of strata in boreholes While this spacing may be adequate if a large number of boreholes is to be drilled, there can be a serious deficiency in quantitative soil information if the size of the area under investigation warrants only a few boreholes The lack of information is particularly noticeable where structures with shallow foundations are proposed Thus it is quite usual for the first sample to be taken just below the topsoil, say from 0.2 to 0.7 m

The next, at the 1.5 m spacing, is from 1.7 to 2.2 m Exactly the same depths are adopted for all the boreholes on the site It is normal to place foundations in clay at a depth of 0.9 or 1.0 m Thus there is no information on soil shear strength and compressibility at and for a distance of 0.8 m below foundation level, probably within a zone where there are quite large variations in soil characteristics due to the effects of surface desiccation (Fig. 1.2)

Where only a few boreholes are to be sunk it is a good practice to adopt continuous sampling for the first few metres below ground level or to stagger the sampling depths where the 1.5 m spacing is adopted.